The present invention relates to an active matrix type liquid crystal display device.
Conventionally, there has been an active matrix type liquid crystal display device shown in FIG. 13. In this liquid crystal display device, a plurality of scanning lines GL1, GL2, . . . and a plurality of signal lines SL1, are arranged so as to intersect each other, and a pixel electrode 4 having a pixel capacitance (equal to liquid crystal capacitance) Cp and a TFT (Thin Film Transistor) element 51 are provided at each intersection of the scanning lines GL1, GL2, . . . and the signal lines SL1, . . . If, for example, the scanning line GL1 comes to have a high electric potential, then the TFT element 51 whose gate electrode is connected to the scanning line GL1 is put in an on (conductive) state to apply a specified signal voltage from the signal line SL1 to a pixel capacitance Cp. Subsequently, if the scanning line GL1 comes to have a low potential, then the TFT element 51 is put in an off (non-conductive) state. The pixel capacitance Cp can be regarded as a capacitor, and therefore, the accumulated electric charges are kept retained to allow the liquid crystals to retain a specified alignment state. By executing this operation on all the scanning lines GL1, GL2, . . . during one frame, a video image of one screen can be produced.
Active matrix type liquid crystal display devices of this kind are widely used as the displays of television screens and word processors.
However, the aforementioned conventional active matrix type liquid crystal display device has the problems as follows.
The liquid crystals to which a voltage is applied have a variety of alignment states depending on the signal conditions of the previous frame. Therefore, the pixel capacitance Cp, which is a capacitor provided by both electrodes that interposes liquid crystals between them, is able to have various capacitances depending on the alignment conditions of the liquid crystals. That is, if the pixel capacitance Cp comprised of the liquid crystal capacitance is charged with a specified voltage determined in accordance with image data, then the quantity of electric charges varies depending on the alignment conditions of the liquid crystals during charging even with an identical signal (voltage).
A reduction in image quality due to this will be described on the basis of a normally-white active matrix type liquid crystal display device that employs twisted nematic (abbreviated to TN hereinafter) liquid crystals. FIG. 14 shows an application voltage-to-transmittance characteristic curve of TN liquid crystals, while FIG. 15 shows an application voltage-to-dielectric constant characteristic curve.
For example, it is assumed that a pixel that has continued the display of white color is made to suddenly display black color. In this stage, the voltage application (charging) of the first frame of the display of black color is effected on the alignment state of the liquid crystals that are displaying white color. As shown in FIGS. 14 and 15, the dielectric constant of the liquid crystals that are displaying white color is smaller than the dielectric constant of the liquid crystals that are displaying black color. Therefore, if a signal voltage of the display of black color is applied to the liquid crystals in the state in which white color is displayed, then the dielectric constant increases according to the response of the liquid crystals. Accordingly, the quantity of electric charges is retained even when the response of the liquid crystals is sufficiently fast, and consequently, the voltage becomes small in the next frame.
As described above, the application voltage runs short when the black color is to be displayed, and consequently, as shown in FIG. 14, the display of the voltage before the achievement of the voltage of the display of black color, i.e., gray color is disadvantageously displayed. When displaying a motion picture, this phenomenon is perceived as an afterimage by the human eye. FIG. 16 shows a change in the application voltage of the liquid crystals when the display of white color is changed to the display of black color, while FIG. 17 shows a change in transmittance. As a result, a step-like response waveform as shown in FIG. 17 occurs, proving the existence of an afterimage.
In order to improve this phenomenon, an auxiliary capacitance is sometimes provided parallel with the liquid crystal capacitance. However, there is a certain change in the capacitance of the liquid crystals at the time of response according to the aforementioned theory, and therefore, the afterimage cannot completely be removed. It can also be considered to reduce the effect of change in the capacitance of the liquid crystals to an ignorable extent by providing a very large auxiliary capacitance. In this case, a reduction in charging rate due to an increase in the electric load and a reduction in the aperture rate of pixel due to the large auxiliary capacitance occur, and therefore, the other demerits are inappropriately large. Therefore, the liquid crystal capacitance and the auxiliary capacitance are generally set to values that are roughly equal to each other, and eventually, the aforementioned afterimage exists.
Accordingly, the object of the present invention is to reduce the afterimage attributed to the change in the dielectric constant (voltage) at the time of response of liquid crystals.
In order to solve the aforementioned problems, the present invention provides an active matrix type liquid crystal display device comprising:
a TFT substrate on which a plurality of scanning lines and a plurality of signal lines are arranged so as to intersect each other and which has a pixel electrode, a memory capacitance and first, second and third TFT elements provided at each intersection of the scanning lines and the signal lines;
an opposite substrate having an opposite electrode; and
a liquid crystal layer held between the TFT substrate and the opposite substrate,
gate electrodes of the first, second and third TFT elements being connected to the scanning lines,
the first TFT element controlled to determine whether or not electric charges are supplied from the signal line to the memory capacitance,
the second TFT element controlled to determine whether or not the electric charges stored in the memory capacitance are supplied to the pixel electrode,
the third TFT element controlled to determine whether or not the pixel electrode is connected to a wiring line of a specified voltage, and
the first, second and third TFT elements being comprised of one n-type MOS element and two p-type MOS elements or constructed of one p-type MOS element and two n-type MOS elements.
An operation when selecting a certain scanning line will now be considered.
In this case, it is assumed that each of the first and third TFT elements is the n-type MOS transistor and the second TFT element is the p-type MOS transistor. The selection of a scanning line is executed by providing a high potential on the scanning line.
If the scanning line is selected with the provision of a high potential, then the first and third TFT elements, which are each constructed of the n-type MOS transistor, are turned on, while the second TFT element, which is the p-type MOS transistor, is turned off. Consequently, the signal voltage from the signal line is applied through the first TFT element to the memory capacitance. The memory capacitance is provided by, for example, electrodes, an insulating film and so on. Therefore, no change in capacitance occurs dissimilarly to the liquid crystals, and this memory capacitance can be charged with a very strictly specified quantity of electric charges with respect to the signal voltage. Simultaneously with this, the pixel electrode is connected to the wiring line of the specified potential through the third TFT element put in the on state, and therefore, the electric charges of the liquid crystal capacitance-are discharged.
On the other hand, the other scanning lines are kept at the low potential in this stage. Then, the first and third TFT elements connected to the other scanning lines are in the off state, while the second TFT element is in the on state. Therefore, the movement of electric charges from the memory capacitance through the second TFT element to the pixel electrode (liquid crystal capacitance) occurs and continues until the liquid crystal response is completed.
The scanning lines are sequentially selected, and the other scanning lines are made to have a low potential.
Through this operation, the liquid crystal capacitance before the charging has already been discharged by the third TFT element, and the dielectric constant has been constant. A specified quantity of electric charges are supplied through the second TFT element to the liquid crystal capacitance of the specified dielectric constant from the memory capacitance of which the capacitance does not change.
Therefore, according to this active matrix type liquid crystal display device, a specified voltage that does not depend on the data of the previous frame can be applied to the liquid crystals even in the case of a motion picture of which the video signal frequently changes, and therefore, the afterimage can be significantly reduced.